Simulating the dynamics and non-thermal emission of relativistic magnetized jets I. Dynamics

Abstract

We have performed magnetohydrodynamic (MHD) simulations of relativistic jets from supermassive blackholes over a few tens of kpc for a range of jet parameters. One of the primary aims was to investigate the effect of different MHD instabilities on the jet dynamics and their dependence on the choice of jet parameters. We find that two dominant MHD instabilities affect the dynamics of the jet, small-scale Kelvin-Helmholtz (KH) modes and large-scale kink modes, whose evolution depends on internal jet parameters like the Lorentz factor, the ratio of the density and pressure to the external medium, and the magnetization and hence consequently on the jet power. Low power jets are susceptible to both instabilities, kink modes for jets with higher central magnetic field and KH modes for lower magnetization. Moderate power jets do not show appreciable growth of kink modes, but KH modes develop for lower magnetization. Higher power jets are generally stable to both instabilities. Such instabilities decelerate and decollimate the jet while inducing turbulence in the cocoon, with consequences on the magnetic field structure. We model the dynamics of the jets following a generalized treatment of the Begelman-Cioffi relations, which we present here. We find that the dynamics of stable jets match well with simplified analytic models of expansion of non-self-similar FRII jets, whereas jets with prominent MHD instabilities show a nearly self-similar evolution of the morphology as the energy is more evenly distributed between the jet head and the cocoon.